For protection against a wide array of diverse pathogens, T cells acquire distinct effector functions in response to each infection. For CD4+ T cells, these effector functions are characterized by the predominant cytokines produced by the effector cells. To date, five different subsets of CD4+ effector T cells have been described, and can be generated from nave CD4+ precursors under controlled stimulation conditions. Similar subsets of functionally distinct CD8+ effector T cells have also been described. While early work in this area indicated that effector T cell differentiation was comparable to the terminal differentiation processes that occur during ontogeny, recent evidence indicates that T cell effector subsets are more plastic in their differentiation status. Not only do some T cells exhibit characteristics of more than one effector lineage at the same time, but additional instances of CD4+ and CD8+ effector T cells acquiring new cytokine profiles over the course of a response have also been observed. For instance, under certain conditions, CD4+ Th17 cells will acquire the capacity to produce IFNgamma, and Th1 cells will become IL-21-secreting Tfh cells. These data suggest that T cell responses can evolve over time, leading to alterations in the effector functions that predominate at different stages of an immune response. Importantly, this process is likely to play a key role in the evolution of autoimmune responses and may also contribute to the pathogenesis of chronic inflammatory diseases. We hypothesize that the transcriptional repressor, Blimp-1, is a critical regulator of T cell plasticity. Blimp-1 is upregulated in activatd CD4+ and CD8+ T cells by a specific subset of cytokines, including IL-2, IL-12, and IL- 4; thus, Blimp-1 is expressed in CD4+ effector Th1 and Th2, but not Th17, cells, as well as in Type I effector CD8+ T cells. We find that Th1 cells generated from Blimp-1-deficient nave CD4+ T cells acquire a multi- lineage molecular program, expressing both Th1- and Th17-specific genes; a similar change in gene expression is seen in Blimp-1-deficient CD8+ T cells following LCMV infection. These data suggest that Blimp- 1 normally functions to repress Th17 and Tc17 differentiation, and further, may be required for effector cells to maintain a highly polarized Typ I subset identity. To test this hypothesis, we will first examine the molecular regulation of Blimp 1 transcription by distinct cytokines to determine the pattern of Blimp-1 expression at different stages of the immune response. We will then determine whether graded expression of Blimp-1 and/or Bcl-6 regulate Type I versus Type 17 effector cell differentiation during the development of a Th17-dependent autoimmune disease and during virus infection. Finally, we will determine whether persistent Blimp-1 expression is required to maintain Type I lineage identity, and whether conditional deletion of Blimp-1 in effector T cells alters their effector functions, and promotes their ability to induce autoimmunity. Together, these studies will determine whether the magnitude and duration of Blimp-1 expression are critical in the maintenance of T cell differentiation states, and whether alterations in Blimp-1 expression during an immune response contribute to the plasticity of effector functions. These data will provide important insights into the mechanisms contributing to autoimmune and other diseases caused by pathogenic T cell responses.